In products such as RF transceivers, it is desirable to integrate as much circuitry as possible on a single, semiconductor chip. This has not been practical for most such products because of the power requirements associated with the transmitter's power output stage. Consequently, the power output stage has usually been constructed as an off-chip, discrete amplifier.
Certain semiconductor products and processes have been thought to be nearly ideal for integrating RF communication systems on silicon. One such process is referred to as SOI (Silicon-on-Insulator). It offers high circuit density, low power operation at low supply voltage, intrinsically high substrate isolation, and other properties that make it very suitable for digital and small-signal RF applications. However, SOI has not been considered as practical for integrating the RF power output stages used by two-way communication products. The problem is heat transfer.
In the typical SOI process, silicon dioxide is used as the insulator over a silicon substrate. The thermal conductivity of the silicon dioxide is much lower than that of silicon. In addition, due to second-order effects, the thermal conductivity of the silicon itself, when placed in thin layers, is less than the its bulk value by as much as 35 percent. These effects cause severe self-heating in large SOI power devices, and limit the amount of power one may generate on-chip. This forces the power amplifier to be off chip, with all of the associated cost and size penalties of discrete components.
Another technology that has been used successfully in small signal applications is referred to as CMOS (Complimentary Metal Oxide Semiconductor). The drawbacks of CMOS for RF applications include its inability to provide good inductors, and its large parasitic substrate capacitance. Consequently, CMOS has not usually been considered a good technology for RF applications.
It has been reported that CMOS may be combined with SOI in certain RF applications involving relatively low power and small signals. See, for example "CMOS-Microwave Wide band Amplifiers and Mixers on SIMOX-Substrates" by Eggert et al., European Solid State Circuits Conference, September 1995, pp 302-305. However, power output stages have not been practical to build using SOI, CMOS or a combination of SOI and CMOS technologies. Consequently, the power output stages of RF products continue to be constructed with discrete components, resulting in RF products that are larger and more expensive than is desired.